The present invention relates generally to improvements in the video display monitor art. More particularly, the present invention seeks to provide circuitry for adjusting various conditions in video display monitors during operation without distortion of the video display.
Color video images are generated in television receivers utilizing picture tubes having red (R), green (G) and blue (B) electron guns. Generally, the electron guns are driven by applying relatively high voltages to their respective cathodes. Due to the frequently high voltages and other effects of relatively heavy use, cathodes and the supporting circuitry are subject to wear over time. When one cathode wears or ages at a different rate than the others, the color tracking of the picture tube, sometimes referred to as color temperature, may be adversely affected. Correspondingly, the color picture quality may be unsatisfactorily degraded. Accordingly, it has been desirable to monitor cathode currents against known standards and provide correction signals to match the cathodes as nearly as possible. This has frequently been done in standard television applications by testing during video blanking intervals. In television applications, this has been generally acceptable because of the fairly long vertical blanking intervals used in standard television signals. Additionally, television receivers have traditionally been overscanned, that is, a percentage of the active raster is off the screen. Thus, sampling of the cathode currents during vertical blanking intervals creates no visual distortion on the screen.
U.S. Pat. Nos. 4,370,674 and 4,342,048 illustrate various circuitry and methods for such CRT stabilization and tracking.
Similar aging effects may occur in color video monitors, and such effects may be more undesirable because of the frequently rigid demands for contrast between text and graphics in such applications. However, monitoring of the cathode currents and the aging effects of the individual cathodes may be more difficult in a color video monitor application, because the CRT is typically underscanned with the entire active raster located within the viewing area of the screen defined by the cabinet bezel opening. Accordingly, traditional techniques may be unacceptable because of potential visible distortion in the display during the cathode current sampling. Also, the active raster typically may not be expanded beyond the bezel opening because of physical constraints associated with the use of color video monitors in computer related applications, e.g., high frequency requirements, limited retrace time, and need for visual video information on all of the active lines of the display field.
Accordingly, it is the primary object of the present invention to generally overcome the deficiencies in the prior art.
It is a further primary object of the present invention to provide a means and method for allowing cathode sampling in a color video monitor while preserving the visual video display.
It is a further object of the present invention to provide a last-line overscan feature in a color video monitor for allowing CRT cathode current sampling while the visual raster trace is off-screen without undue negative impact on the vertical retrace time.
It is still a further object of the present invention to provide for an improved color video monitor in which aging affects may be continually monitored so as to allow for the preservation of the desired color tracking between R, G and B cathodes.